Propeller and the like



NOV. 20, 1934. 5 SELMAN PROPELLER AND THE LIKE 2 Sheets-Sheet 1 Filed Dec. 1, 1933 NOV. 2 1934- G. s. SELMAN 1,981,392

PROPELLER AND THE LIKE Filed Dec. 1, 1933 2 Sheets-Sheet 2 K. J J e/W? vveMfalf Patented Nov. 20, 1934 UNITED STATES.

PROPELLER AND THE LIKE.

George Sidney Selman, London, England, assignor to The Manganese Bronze & Brass Company, Limited, London, England Application December 1, 1933, Serial No. 700,591 In Great Britain December 3, 1932 2 Claims.

This invention relates to propellers and the like, and has for its object the provision of a particularly efficient form for such devices which shall at the same time be substantially free from 5 the disadvantages of erosion and cavitation.

A most important part of a propeller section is the portion of the back of the blade between the maximum thickness and the leading edge, and an object of the present invention is to form this part so that maximum efiiciency may be obtained with freedom from erosion and cavitation.

Experiments with propeller blades having sections of various thicknesses have shown that the highest efficiency can be obtained if the camber ratio of the blade section, viz., the ratio of thickness to width, is in the neighbourhood of .075 when the maximum thickness is situated 29% of the length of the blade section, 1. e., .29 L from the leading edge. Subsequent experiments when pressures were plotted as contours round the profile have shown that thicker sections with a bluffer nose cause serious instability of flow and low efliciency, in addition to which propellers may erode on the driving face; again thinner sections having finer noses are less eflicient, and may be subject to erosion on the back at high velocities.

The invention consists in a propeller blade or the like wherein the shape of the curve of the back of the blade from the point of maximum thickness to the leading edge of the blade is similar for all the said cross sections, the ratio of the height of the curvature above a line passing through the leading edge of the blade and coincident with or parallel to the helical length line contained in the helical surface of the working blade face, that is the driving face at any given section to the distance between the ordinate of maximum thickness and the leading edge of the blade at the section in question preferably being 0.26 with a tolerance of $0.02.

The invention also consists in a propeller blade or the like according to the preceding paragraph wherein the maximum thickness measured at right angles to the helical length line referred to below at any cross section of the blade lies at a distance from the leading edge of the blade which is from 25% to 29% of the width of the blade at that section, that is, of the length measured along the helical length line contained in the helical surface of the working blade face at that section.

The invention also consists in propeller blades or the like substantially as herein described with reference to the accompanying drawings which are not necessarily to the same scale.

Referring to the accompanying drawings: Figures 1 and 2 respectively show sections of a propeller according to the invention in full lines with corresponding sections of a normal propeller in dotted lines, L being the expanded length measured along the helical length line referred to above;

Figure 3 shows a longitudinal section of a blade along the points of maximum thickness.

Figure 4 shows a series of developed blade sections in the flat together with the expanded blade outline formed by joining the ends of all such sections.

Figure 5 is an expanded blade outline similar to that of Figure 4 showing the angular relationship of the propeller blade to the hub in one form;

Figures 6 and 7 show two cross sections of a blade taken at different radii and having different thicknesses.

Figure 8 is a line drawing made from a photograph of a propeller constructed in one form of the present invention.

In the form of the invention illustrated in Figures 1 to 4 and 8, the thicknesses of the blade are adjusted so as to give a constant ratio of thickness la is the distance of the maximum thickness from the leading edge measured as referred to above and. to is the height of the maximum ordinate above the under side of the nose as referred to above and below, L being the length of the section as shown for instance in Figure 1. In the case of ordinary cargo boats with the propellers having comparatively narrow blades according to the present invention, the preferred limits of 16 are .25 L and .29 L associated with camber ratios of .064 and .075.

There are two practical obstacles to the achievement of this, the first that for strength purposes using ordinary material one is usually forced to adopt a camber ratio greater than .075 on the sections near the boss. If such a camber ratio were used for the sections near the tip the propeller would be unduly heavy.

The blade outline is, therefore, made rather more narrow towards the tips and wider at the root than a propeller having a normal elliptical outline, in order to modify the camber ratio in the direction required, i. e. lower at the root and higher at the tips. This is, however, insufficient, and so the following expedient is adopted:

Where the camber ratio of a propeller according to this invention is greater than .075 the nose of the section is lifted up an amount equal to the thickness of blade minus .075 L, with the maximum ordinate at .29 L from the nose as shown in Figure 1. Further up the blade, or close to the tip, the camber ratio becomes lower and the turn up less. When the camber ratio is equal to .075 L, an approximately flat face is given to the section and the maximum ordinate is at .29 L from the leading edge. Closer to the tip the higher velocities experienced are likely to cause breakdown in the flow, and erosion. Againit is not desirable to place the maximum ordinate closer to the nose than .25 L in normal circum stances but it may be necessary when very wide blades are adopted. When thethicknesses re quired for strength purposes give a camb'er'ratio less than .064, the thickness is increased until the ratio reaches .064, and the maximum ordinate placed .25 L from the leading edge as shown in Figure 2. Wards the tip which is shown in a somewhat exaggerated form in Figure "3 referred to below, and this thickening we regard as of importance.

The maximum thicknesses arrived at must be faired in to one another, and this gives at some points camber ratios intermediate between .075 and .064, in which case the maximum ordinate is shifted to give a constant shape to the curved back of the section at the nose with a constant value of t e 1e of .26 approximately.

Figure 3 shows a blade outline as referred to above and maximum thickness section arrived at by the application of the above. This figure also enables a convenient summary to be given of a method of constructing a blade according to the present invention and also a method of examining a constructed blade to determine'whether it is in accordance with the present invention or not.

From the constructional point of view, and considering for convenience the bottom section shown in the right-hand part of Figure 3, the length L is determined from the requirements of the strength or area of the blade required at that part; the place of maximum thickness as referred to above is preferably to 29% of this length from the leading edge, the figure varying with the design of the blade and in the particular section chosen being 0.29 L. That determines the position of the line t. The distance te is measured down the line t from the top of the section, the amount being, as explained above, 0.26 times the distance between the line If and the leading edge of the blade with a tolerance of i002. A horizontal line drawn through the point thus found determines where the top and bottcmpurves of this section meet, but in this connection reference should be made to the statement below dealing with the untrimmed nose. The sections are made, for instance. in thin wood and applied to the struck driving face of the propeller mould being bentinto curves in accordance with the appropriate radius from the centre of the propeller. The shape of the curve between the place of maximum thickness and the leading edge of the blade is preferably approximately as shown.

From the point of view of testing a completed blade the distanceL can be measured. The 100- v sition of maximum thickness can be found, but it must be borne in.mind that the maximum thickness of the'blade as measured on a completed propeller is not the depth of the section This causes a slight thickening toused at that place in constructing the propeller blade. The maximum thickness can be measured and the distance between the location of maximum thickness and the forward edge of the blade can be measured. It can then be determined by calculation whether the ratio 0.26i0.02 referred to above is present or not.

Fig'ure A shows a blade outline constructed as f described with reference to Figure 3 and shows the angle ACB which is the angle through which the blade isswept back from the radial position. Generally the total length of the propeller boss is taken up by the blade width at the root and largely determines its maximum width.

The actual blade thickness is determined by stress calculations and depends on the material used in the propeller and upon the width of the blade, but the latter is generally restricted by the length of boss available.

but a depth normal to the driving face of the The shape of the curve of the back of the blade is determined approximately, by dividing the length of entrance Ze into several parts and erecting ordinates bearing the correct relation to the thickness of entrance te as shown in Figure 5.

In Figures 1, 2 and 5, the leading edge of the blade has been shown radiussed. In practice the nose of the blade is treated as though it were fiat and formed part of a line such as my (Figure 1). The curve of the back of the blade between the nose and the place of maximum thickness starts from the top edge of this flat part. The thickness of. the untrimmednose which is approximately .18 times .075 L is finally trimmed to a radius. In practice I often increase the thick- T es ness of the untrimmed nose when the camber ratio is greater than .075 up to a thickness equal to .18 t and the extra thickness is added to the under side of the section as shown in Figure l.

Propeller blades, especially those of marine 3 propellers, are usually designed with reference to sections of the blade cut by cylinders concentric with the axesof rotation of the propeller. The

term cross sections in this specification refers to the aforesaid sections when developed. I

I claim:

1. A screw propeller blade or the like wherein that portion of the back of the blade between the position of maximum thickness and the leading edge is defined by geometrically similar curves for all blade sections,and wherein the ratio of the maximum ordinate of the said curves measured from a line passing through the leading edge of the blade, and coincident with or parallel to the helical length line contained in the helical surface of the driving face of the blade, to the distance between the ordinate of maximum thickness and the leading edge, is .26 with a tolerance of $0.02, and wherein the curve forming the back of the blade between the end of the maximum ordinate and the leading edge of the blade is substantially elliptical.

2. A screw propeller blade or the like as claimed in claim 1 wherein the ordinate of maximum thickness measured at right angles to the aforesaid helical length line at any cross section of the blade lies at a distance from the leading edge of the'blade which is from 25% to 29% of the width of the blade at that section.

GEORGE SIDNEY SELMAN. 

